Chemical Thermodynamics of Tin - Volume 12 - OECD Nuclear ...

274
A Discussion of selected references
and it was attempted to determine thermodynamic quantities of Reaction (A.18)
Sn 4+ + H 2 (g) Sn 2+ + 2 H + .
(A.18)
The Sn 4+ /Sn 2+ electrode potential was found to be 0.154 V, which is 8 mV
more positive than that given by [1914FOR/BAR].
For a re-evaluation of Huey and Tartar’s measurements and the consecutive
SIT analysis it is essential to convert the compositional data “Sn, mml. per 1000g. of
soln.” into molalities. The somewhat cryptic expression – Sn, mml. per 1000g. of soln.–
has been translated into millimole (Sn) tot (SnCl 2 + SnCl 4 ) per 1000g of solution. The
symbol μ and the unit mol·kg –1 has been provisionally assigned to this quantitiy. The
above mentioned conversion has been based on the following relationships:
1) Tin has been dissolved in HCl and partially oxidised by Cl 2
μ Sn tot
= μ SnCl + μ
2 SnCl .
4
2) The molar ratio n(Sn(II))/n(Sn(IV)) can be expressed by μ or m
μ SnCl 2
+ μ SnCl = m
4 SnCl + m
2 SnCl .
4
3) Mass fractions can also be expressed by μ or m
w SnCl 2
= μ SnCl M
2 SnCl /1000
2
= mSnCl
M
2 SnCl /( m
2 SnCl M
2 SnCl + m
2 SnCl M
4 SnCl + m
4 HClM HCl + 1000).
The column “Barometric pressure, mm at 0 °C” was treated as follows. The
water activities in HCl solutions listed e.g. in Table B-1, were used to derive a quadratic
function of the equilibrium vapour pressure p HO over the pertinent hydrochloric acid
2
solutions m HCl . The pressure of the hydrogen p H was taken as the barometric pressure
2
given in Table I, column 5 [1934HUE/TAR] less the calculated value of p HO.
2
and thus
The activity coefficients of the hydrogen ion were calculated according to
o
m
'(HCl)
log10 γ ± ,HCl
= − D + ε(H + , Cl – )·m HCl ,
−1 −2
H
E = E(Sn 4+ /Sn 2+ ) − (k/2) log 10{ mSn(IV) pH m
2 Sn(II) a + } was obtained.
Figure A-9 shows that at m HCl ≥ 0.53 m
within the scatter of the experimental data.
E is independent of m (Sn)tot ,
o
m
'(HCl)
So far the results of [1934HUE/TAR] were confirmed. A problem arises,
however, due to Huey and Tartar’s method of extrapolation to m HCl = 0. Probably the
o
o
function E
m
'(HCl)
= E
m(HCl)
+ b m
HCl
was used, although this was not stated explicitly,
ο
see Figure A-10. The value, E (Sn 4+ /Sn 2+ ) = 0.154 V, given by [1934HUE/TAR] has
essentially been reproduced by this kind of curve fitting.
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012